Developments in the techniques of molecular biology and transformation have allowed the production of transgenic cells, plants or animals with various desirable traits, such as, e.g., resistance to insects and diseases, such fungal or microbial pathogens, tolerance to herbicides or value-added traits. These desirable traits are mainly obtained by overexpression of a transgene in the cells, plants or animals. However, in some cases, it is also desirable to modify cells, plants or animals so that the expression of a particular gene is altered to create cells, plants or animals with desirable phenotypes or properties of commercial interest. Current methods to alter the expression of a gene usually rely upon techniques of sense (see e.g. Jorgensen et al. (1996) Plant Mol. Biol. 31: 957–973) or antisense suppression (see e.g. Green, P. J. et al. (1986) Ann. Rev. Biochem. 55:569–597). For example, sense suppression of a chalcone synthase gene in Petunia results in flowers with altered pigmentation and antisense suppression of a polygalacturonidase gene in tomato leads to delayed fruit ripening. Unfortunately, these methods are often variable and unpredictable in their ability to alter gene expression, and in many cases a complete disruption of the particular gene activity is not achieved. Other methods to alter gene expression include the use of catalytic ribonucleotides or ribozymes (see e.g. U.S. Pat. No. 4,987,071), which can be technically challenging, or homologous gene disruption (see e.g. Paszkowski et al. (1988) EMBO Journal 7: 4021–4026), which although the most desirable genetically, is unfortunately often not efficient enough with currently available techniques to be routinely used for such purposes.
Another area of deep interest is resistance or tolerance to viruses. Viruses affect most living organisms. In crops, large proportions of the harvest may be lost due to virus infections. Farm animals are also often infected by viruses and must sometimes be slaughtered to prevent spreading of the disease leading to dramatic economic consequences. Companoin animals are also affected by viruses, and, finally, viruses infect humans causing a lot of suffering. Although treatments against viruses have been developed, they are very often either extremely expensive or of limited efficiency.
There is therefore a long-felt but unfulfilled need for novel methods and compositions allowing one to effectively and predictably alter the expression of a gene to obtain cells, plants or animals with improved and commercially important properties. In particular there is a long-felt but unfulfilled need for such methods that provide resistance or tolerance to viruses.